Quartz crystals are widely used to provide reference frequencies in electronic oscillators. The frequency at which a quartz crystal resonator vibrates depends on its physical dimensions. Moreover, changes in temperature causes the quartz crystal to expand or contract due to thermal expansion and changes in the elastic modulus of quartz. The physical changes in turn change the crystal oscillation frequency. Although quartz has a very low temperature coefficient of frequency, temperature changes are still the major cause of frequency variation in crystal oscillators.
Oven controlled crystal oscillators (“OCXO”) are frequency reference devices where the quartz oscillator is placed inside a temperature controlled oven. The oven is provided to maintain the oscillator at a constant temperature in order to prevent changes in the frequency due to variations in ambient temperature. This type of oscillator achieves the highest frequency stability possible with a quartz crystal. OCXOs are typically used to control the frequency of radio transmitters, cellular base stations, military communications equipment, and devices for precision frequency measurements, for example.
For OCXOs, the oven is a thermally-insulating enclosure that contains the crystal and one or more electrical heating elements. Since other electronic components in the oscillator circuit are also vulnerable to temperature drift, usually the entire oscillator circuit is enclosed in the oven. For these devices, a temperature sensor, such as thermistor, will be provided to monitor the oven temperature and a closed-loop control circuit will be provided to control the power to the heater to maintain the oven at the precise target temperature. Since the oven operates above ambient temperature, the oscillator usually requires a warm-up period of several minutes after the power has been applied. Moreover, the frequency of the device will not have the full rated stability during this warm-up period.
Although existing OCXOs generally provide good stability (e.g., typically better than 100 parts per billion (“ppb”) over a specified temperature range), these devices also have several shortcomings. First, a typical quartz crystal is fairly large, which, turn, makes the final OCXO devices quite large. Since the manufacturing cost of the timing device is proportional to the size, the larger OCXO size is not preferred. Second, the long thermal time constant for heating and cooling leads to a very long start-up time. For example, it typically takes several minutes to stabilize the oven at the target temperature. Third, the power needed to maintain the oven temperature is fairly large. For example, a typical OCXO consumes over 1 watt to heat the oven. Finally, due to temperature gradients in the oven, the crystal temperature is not constant but may change by +/−1 K over an ambient temperature range of −40 to 85 C.